Method for straightening a rail and straightened rail

ABSTRACT

The steel rail is submitted to a tensile stress exceeding the conventional 0.2% offset yield strength of the steel, up to a stress value corresponding to a total plastic deformation of the whole rail.

The invention relates to the finishing of rails and more particularly tothe relaxation of stresses and the straightening of heat treated,standard grade steel or extra-hard alloyed rails.

After rolling, the hot rail, which is then very sensitive todeformation, is exposed to a series of handling operations andoperations such as transport on roller conveyors, cutting and transfers,which can create deformations. Their cooling is also a source ofsubstantial deformations, despite all the precautions that can be takento minimise or avoid them. Irregular cooling of the different parts ofthe rail the profile of which is asymmetric with respect to its two mainplanes has the effect that the rail coming from the cooling bedsexhibits a more or less marked camber, which depends on the coolingconditions. The lengths of the fibres of the head, the web and the footof the rail are unequal. Whatever precautions are taken to avoid orminimise the camber resulting from cooling, it is impossible, inindustrial production, to obtain, on leaving the cooling beds, 100% ofrails sufficiently straight to be delivered in that state to thecustomers. The inevitably irregular cooling of the rail because of theasymmetrical profile of the rail is, on the other hand, a source ofresidual stress which can promote the propagation of cracks when therail is installed in the track, principally with extra-hard rails usedon heavily loaded tracks (for example, mine tracks or heavy haultracks).

The heat treatment of rails, applied to all or a part of their profile,before their passage through the cooling beds, or the controlled coolingof rails in pits, increase the risks of substantial deformations andresidual stresses. The less severe specifications applicable to theproduction of rails no longer allow them to be used in the straightnesscondition that they present when they leave the cooling beds. It isabsolutely necessary to straighten them. In all straightening methods,it is necessary to subject the metal to a stress greater than theelastic limit, so as to treat it in the plastic deformation region, atleast locally.

Two types of straightening machines have been and are still being usedaccording to the prior art. The older is a gag press in which a portionof rail that is to be straightened is laid upon two supporting anvils. Apress piston, which moves vertically, on the free end of which is fixeda liner piece adaptable to the dimension of the rail to be straightened,deforms by pressure the portion of the rail, to give it an inversebending. Laterally located anvils and pistons, allow, by the sameprinciple, the lateral straightening of rails. The press operatordetects visually the parts of the rail that need straightening andchecks with a ruler, after each stroke of the press, the straightnessobtained. This method of straightening, which requires an experiencedoperator, proceeding by multiple press strokes on portions of the rail,is rough and expensive. The result obtained does not meet all therequirements of a modern rail system.

In general, it is used today only as a complement to the straighteningwith roller straighteners that belong to the second type ofstraightening machinery. This machine straightens the rail in one or twoinertial planes of the latter and comprises generally between 5 and 9rollers. The rail is subjected alternately to bending deformations inopposite directions. The driven upper rollers draw the rail along andcause it to undergo, with the lower rollers, which are not driven,deformations in alternating opposite direction. In the triangle formedby the three first rollers, the rail is subjected to an a priori setdeformation, which is not related to the actual deformation of eachindividual rail. In the second triangle formed by the second, third andfourth rollers, the rail is subjected to a deformation inverse to thefirst. The fifth roller and those following have the function, byappropriate alternating deformations, of making the rail straight. Theends of the rail are not straightened over a certain distance whichcorresponds to the axial spacing of the rollers. These ends must then bestraightened by a gag press. The roller straightening method usingrollers puts certain fibers of metal successively in tension and incompression. After a roller straightening, the web of the rail is inlengthwise elastic compression, while the head and the foot are inlengthwise elastic traction. These internal tensions due to the rollerstraightening. Regardless of the initial state of straightness of railsafter the cooling stage, all rails are subjected in roller straighteningto substantial deformation, leading to the following disadvantages.

sensible shortening of the rail;

reduction in the height of the rail profile;

increase of the width of the head and of the foot of the rail;

systematic differences in rail dimensions between the ends of the railsnot worked by the rollers and the body of the rail which has been soworked;

frequent necessity to finish the straightening of the ends on a gagpress which makes slight flats on the ends, and therefore rendersimpossible a perfect continuity of straightness with the main part ofthe rail;

systematic generation, in all rails, of stresses which can promote thepropagation of cracks;

risk of forming brittle fracture zones in the interfaces of the web withthe foot or the head. These fracture zones, being internal, areinvisible and pose a very serious risk of a potential accident;

risk of creating on the head of the rail of sinusoidal waviness ofvarious amplitudes due to hard-to-avoid eccentricities of the rollers,waviness which can cause more or less serious disturbance on the trackwhen the train speed is important.

The roller straightening methods eventually used with gag presses permitthe present specifications applicable to the manufacture of rails to besatisfied only at the cost of close and expensive control. The UIC 860specification, for example, prescribes in regard to straightness, amaximum permissible deflection of 0.7 mm over 1.5 m for the end of therails, the straightness being judged by the eye for the body of the bar.For rails intended for high speed train tracks on which trains travel ata regular speed of 260 Km/h (tracks on which a speed of 380 Km/h hasbeen achieved) the UIC 860 specification is augmented by the followingsupplementary specifications:

the maximum permissible deflection is of 40 mm for 18 meter long railsand of 160 mm for 36 meter long rails;

the vertical amplitude of the waviness on the tread of the head shall beless than 0.3 mm;

the horizontal amplitude of the transverse waviness of the head of therail shall be less than 0.5 mm;

alignment of the ends with the body of the bar, in the verticaldirection, defined by a maximum permissible deflection of 0.3 mmmeasured with a 3 meter long ruler resting on the tread surface at theends.

The meeting of these supplementary standards, which requires the rollerstraighteners and the gag press to be operated up to the limit of theirpossibilities, increases the cost of the straightening operation.

It has also been proposed to stretch straighten any metal profiles (seeFrench Pat. No. 573/675 of Feb. 23, 1923). According to this process,any profile, more or less deformed, is straightened by stretching inorder to regularly extend its fibers until the elastic limit of themetal is reached or even exceeded. It is known also that stretching ametal increases its hardness while reducing by substantial deformationits characteristics of ductility and resilience. Now, it is principallythe tenacity which is important for a rail. This is probably essentiallythe main reason that up to now has prevented those skilled in the artfrom using the stretching method for straightening rails.

For economic reasons, rails are being made more and more of hard steelwhich is rather brittle due to its content of hardening elements, suchas carbon for instance. It has been determined that in this kind ofrail, the speed of propagation of fatigue cracks is very high. It isknown that fatique can develop whenever the residual stresses reach ahigh level. It can be seen from the following table that for rollerstraightened rails, the internal stresses or tensions reach thefollowing levels:

    ______________________________________                                        Type of steel                                                                              breaking load internal stress                                    ______________________________________                                        UIC Standard 700 to 900 N/mm.sup.2                                                                       100 N/mm.sup.2                                     grade steel                                                                   UIC Naturally                                                                              900 to 1000 N/mm.sup.2                                                                      200 N/mm.sup.2                                     hard steel                                                                    UIC Extra-hard                                                                             1100 to 1200 N/mm.sup.2                                                                     300 N/mm.sup.2                                     steel                                                                         ______________________________________                                    

The invention which proposes to eliminate the disadvantages of the priorart methods of straightening rails and avoid the need for acomplementary straightening with a press, has as its object:

the production of rails free from bends;

the guaranteeing of a continuity in the straightness between the endsand the body of the rail, by the elimination of all flats at the ends;

guaranteeing the absence of periodic waviness on the tread surface ofthe head;

elimination of the risk of brittle fracture in the regions that connectthe web with the foot and the head;

not to create untoward internal tensions at the time of thestraightening operation;

the reduction of internal tensions introduced into the rail by theoperations preceding the straightening (heat, cooling treatments).

To achieve these objects, the invention proposes:

to submit the steel rail as shown per se to a tensile stress exceedingthe conventional 0.2% offset yield strength of the steel up to a stressvalue corresponding to a complete plastic deformation of the entirerail.

By virtue of this fully plastic deformation of the rail by stretching,no residual stress is created by the operation of stretch straighteningand the pre-existing residual strains are relieved.

For the known qualities and grades of steel, whether heat treated ornot, it was discovered that the values of lengthwise residual stressesare lower than +/-100 N/mm² for grades of rail steel having a tensilestrength Rm>1000 N/mm² and lower than +/-50 N/mm² for grades of railsteel having a tensile strength Rm≦1000 N/mm² as soon as the plasticdeformation by stretching of the rail corresponds to a residualelongation of the order of 0.27%.

Put another way, a residual elongation of the rail of 0.3% after releaseof the stretching load guarantees the results stated above. Thereduction of the residual internal stress of the rail to a low valueimproves the tenacity and the fatigue resistance of the rail. In effect,when the rail is positioned in the track, it is subjected inter alia tothe stresses due to the long welded lengths of rails and to those due totraffic.

So long as the combination of these stresses does not exceed theendurance limit of any possible incipient cracks pre-existing in therail, it will not lead to its fracture, whence it is of interest to haverails with residual internal stresses as weak as possible.

It has been discovered that the residual stresses cannot be reducednoticeably further once the whole of the material constituting the railhas undergone a total plastification. Accordingly, it is not necessaryto submit the rail to a stretching load giving a value of residualelongation greater than 1.5%.

The invention aims also to provide straightened rails characterized by avalue of residual internal stress lower than +/-100 N/mm² for grades ofrail steel having a tensile strength Rm>1000 N/mm² and lower than +/-50N/mm² for grades of rail steel having a tensile strength Rm≦1000 N/mm².

The characteristics and advantages of the invention will be evident fromthe following description of preferred embodiments. The descriptionrefers to the annexed drawings of which:

FIG. 1 shows a section of a rail with an indication of its constituentparts, of its neutral plan XX' and of its vertical plane of symmetryYY';

FIG. 2a is a perspective view of a rail as it leaves the cooling beds;

FIG. 2b is a side view of the same rail;

FIG. 3 is a stress-strain diagram of steel, showing the stress curveproduced as a function of the elongation effected;

FIG. 4 shows, for a rail leaving the cooling beds, a diagram of thereduction of residual stress in the different constituent parts of therail as a function of the level of residual elongation E;

FIG. 5 shows in its upper inset part a section of rail with a saw cut oflength L used for a test to establish the presence or otherwise ofinternal stresses, and, in its main part, a diagram showing the resultof the empirical comparison of the state of residual stress by sawingthe web and measuring the deviation of the head at the ends of railswhich are unstraightened, roller straightened and straightened accordingto the invention;

FIGS. 6a and 6b each show the plane of fracture of a naturally hard railB of UIC roller straightened according to the prior art (FIG. 6a) and arail of the same grade straightened according to the invention (FIG.6b), FIG. 6b showing that the fatigue crack before fracture in the railstraightened by stretching is longer than that of the rollerstraightened rail which presents a clearly more accentuated brittlecharacter;

FIG. 7 shows the curves 11 and 12 of cracking compared with thepropagation of the crack in a test of alternating flexure carried out inextra-hard grade alloy rails (UIC naturally hard, Rm<1100 N/mm². It isseen here that the fatigue resistance of the stretch straightened rail(curve 12) is superior to that of a roller straightened rail.

FIGS. 8a-8b-8c-8d show the fracture surfaces of four samples of a railof extra-hard alloyed steel (Rm≧1080 N/mm²) respectively rollerstraightened, stretch straightened, not straightened (straight from thecooling bed) and first roller straightened, then stretch straightened.It is seen here that the stretching method of the invention eliminatesany trace of brittleness in the cracks;

FIG. 9 shows the curves of cracking for the samples of rail of FIGS. 8a,8b, 8c and 8d.

A rail 1 leaving a cooling bed presents a warped curve (FIGS. 2a and b).The lengths of the fibers constituting the head 2, the web 3 and thefoot 4 of the rail 1, being respectively the fibers CC', AA' and PP',are thus unequal. The principle of the invention is to submit the railto a stretching load at each end which puts all the fibers under theeffect of a stress sigma (σ) which exceeds the conventional 0.2% offsetyield strength indicated by Rp 0.2 (FIG. 3), so as to take up the samelength in the fully plastic domain of the rail steel underconsideration. The amount of elongation necessary for this operationshould be greater for the least stretched fiber than the amount ofelongation corresponding to the initial drop in the load/elongationcurve marking the beginning of the plastic domain of the steel. There isthus applied to the rail to be straightened a tensile load exceeding theyield strength so as to obtain, after releasing the load, a permanentelongation of at least 0.27%. This small residual elongation permits theproduction of straight rails, with less damage to the material than whenit is roller straightened. The camber in the rail not being alwaysregular along the length of some bars, one can encounter local radii ofcurvature smaller than the global radius of curvature. A residualelongation of the order of some tenths of a percent allows the removalof the shorter bends and, a fortiori, the longer bends. The existence oftensions or internal stresses coming from cooling implies inequalitiesin the lengths of the fibers of the rail. The straightening by plasticelongation of all the fibers and by preferential plastic elongation ofthe shorter fibers leads to a relaxation of residual internal stressesin the steel. FIG. 4 shows an example of the evolution of residuallongitudinal stresses as a function of the amount of residual elongationfor a rail of standard grade. The graph of FIG. 4 shows as the abscissathe residual elongation ε and as the ordinate the residual longitudinalstress σ (-for compression, +for tension) in N/mm². The curve 5represents the residual stress in the foot and the curve 6 that in thehead of the rail. It is shown that the residual stress remains constantand high as long as the tensile load applied to the rail is in theelastic domain of the steel (value of ε˜0.185%) and that said residualstresses diminishes regularly beyond the elastic domain to reachconstant minimum values from a residual elongation of the order of0.27%.

It is readily understood that the domain of residual elongationcomprised between the conventional yield strength (ε=0.2%) and theminimum values of residual stress (here σ˜10N/mm² for ε≈0.27%) is aregion of uncertainty and is therefore to be avoided and that as soon asthe minimum value of residual stress is reached (as soon as ε≈0.27% or0.3%) an increase in residual elongation does not produce any furtherappreciable improvement in this respect, except for the increase of theyield strength by the effect of strain-hardening, said elevation of theyield strength can be carried out as desired: for example, for a UIC Anaturally hard grade of steel or for a AREA grade, the elevation of theyield strength is of the order of 100N/mm² per 1% of supplementaryresidual elongation.

In other words, a residual elongation of 0.3% is sufficient in this caseto remove the residual stresses, or to reduce them by a factor of theorder of 10 to 1. The values measured with the so-called method ofcutting confirmed by the so-called trepan drilling method, of theresidual stresses of the rails designated by references 0.73 D 09, 236 D23 and 150 C 13 stretch straightened with the method of the invention,and those of the roller straightened rails designated by the references073 B 10, 236 D 23 and 150 C 13, all said rails having been producedclose together, from the same heat and cooled close together on thecooling beds, are given below in tables I to III.

                                      TABLE 1                                     __________________________________________________________________________    Roller straightened   Stretch straightened at 0.7% of                         Rail 073 B10          residual elongation Rail 073 DO9                        σ max in                                                                             σ max in                                                                         σ max in                                                                       σ max in                                   compression  traction                                                                           Total                                                                             compression                                                                          traction                                                                           Total extent                                N/mm.sup.2   N/mm.sup.2                                                                         extent                                                                            N/mm.sup.2                                                                           N/mm.sup.2                                                                         of stresses                                 __________________________________________________________________________    Principal                                                                           -260   +230 490        +40  40                                          stress σ.sub.1                                                          in the                                                                        lengthwise                                                                    direction                                                                     Principal                                                                           -200    +50 250 -10    +30  40                                          stress σ.sub.2                                                          in the                                                                        vertical                                                                      direction                                                                     __________________________________________________________________________

                                      TABLE II                                    __________________________________________________________________________                          Stretch straightening                                                                         Stretch straightening at                Roller straightening  at 3% of residual elongation                                                                  0.5% of residual elongation             Rail 236 D 23         Rail 236 D 23   Rail 236 D 23                           σ max in                                                                             σ max in                                                                         σ max in                                                                       σ max in                                                                         σ max in                                                                       σ max                                                                        Total                       compression  traction                                                                           Total                                                                             compression                                                                          traction                                                                           Total                                                                             compression                                                                          traction                                                                           extent of                   N/mm.sup.2   N/mm.sup.2                                                                         extent                                                                            N/mm.sup.2                                                                           N/mm.sup.2                                                                         extent                                                                            N/mm.sup.2                                                                           N/mm.sup.2                                                                         stress                      __________________________________________________________________________    Principal                                                                           -140   +240 380 -20    +45  65  -10    +30  40                          stress σ.sub.1                                                          in the                                                                        lengthwise                                                                    direction                                                                     Principal                                                                           -150    +30 180 -40    +10  50  -10    +20  30                          stress σ.sub.2                                                          in the                                                                        vertical                                                                      direction                                                                     __________________________________________________________________________

                                      TABLE III                                   __________________________________________________________________________    Roller straightening  Stretch straightening at 1% of                          Rail 150 C13          residual elongation Rail 150 C13                        σ max in                                                                             σ max in                                                                         σ max in                                                                       σ max in                                   compression  traction                                                                           Total                                                                             compression                                                                          traction                                                                           Total extent                                N/mm.sup.2   N/mm.sup.2                                                                         extent                                                                            N/mm.sup.2                                                                           N/mm.sup.2                                                                         of stress                                   __________________________________________________________________________    Principal                                                                           -143   +282 425 -21    +10  31                                          stress σ.sub.1                                                          in the                                                                        lengthwise                                                                    direction                                                                     Principal                                                                            -89    +26 115 -27     +8  35                                          stress σ.sub.2                                                          in the                                                                        vertical                                                                      direction                                                                     __________________________________________________________________________

Summing up, it appears that for a residual elongation of 0.3 to 1%, thelevel of residual stresses is at least 5 to 10 times less with thestretch straightening method than with the roller straightening methodand that the scattering of the values of residual stress measured forstretch straightened rails is five times less than that measured forroller straightening rails. These experimental results were verified bystress measurements made with different methods in differentlaboratories (SACILOR, IRSID).

The relaxation of the residual internal stresses is such that thelaboratories saw no significant differences between the level of stressof stretch straightened rails and the level of stress of the materialsthat were stress relieved to serve as references in the calibration ofstrain gauges. For example, in roller straightened rails one findsrather strong compression stresses, in the lengthwise direction as wellas in the vertical direction, in the web and in the portions thatconnect it to the head and foot, these stresses being balanced,particularly in the lengthwise direction, by strong tensile stresses inthe head and the foot. With stretch straightened rails, the residualstresses are very markedly weaker and much more uniform. It should bepointed out that the values of stress measured by the cutting method(method so-called of YASOJIMA and MACHII (1965) used, inter alia, by theOFFICE of RESEARCH and TESTING of the UIC in its study C53 "Residualstresses in rails") are confirmed in a satisfactory way by the so-calledtrepan drilling method. An empirical verification of the relaxation ofinternal stresses due to the stretch straightening has been made bymeans of a test which consists of separating the head from the rest ofthe profile and measuring its deviation f at its end in proportion tothe advance L of the saw cut (method shown inset in the upper part ofFIG. 5). The results of this test performed on a UIC 60 NDB rail areshown in the graph in FIG. 5, of which the abcissa indicates the lengthL in mm. of the saw cut and the ordinate shows the separation ofdeviation f in mm. of the sawn off head from the rest of the stump ofthe rail at the end thereof.

The curve 7 shows that a roller straightened UIC 60 NDB rail presents aseparation f of the head of 2 mm for a saw cut of length L of 500 mm andthe curve 8 shows for a same not straightened rail a separation whichvaries betwen 0 and 8/10ths of a mm. The curves 9 and 10 show thatstretch straightened rails at 0.3 and 1% of residual elongation presenta separation f respectively of 2/10ths and -1/10th of a mm (slightclosing together) for a saw cut length L of 500 mm. There is shown to bean improvement in the value of f of the order of 1 to 10 in favour ofthe stretch straightening method of the invention. A minimal residualelongation of the order of 0.3% seems to be necessary to achieve amaximum relaxation of the internal stresses and it does not seem that anelongation greater than 1.5% offers any supplementary advantages.

The fact of stretching a rail beyond its conventional yield strengthRP₀.2 might have given rise to a fear of damaging material in such a waythat the damages would accelerate the propagation of eventually existingtransverse fatigue cracks. A fatigue test by flexion at 4 points hasshown that it is not so. The test consists in submitting a rail samplepre-notched in the head to an alternate flexion over a base length of1.400 m at a frequency of 10 Hertz under a load of the order of 14tonnes during a period for opening a crack and of 9 tonnes during theperiod of crack propagation, the load being applied to the head at twopositions spaced by 150 mm situated symmetrically on each side of thecentral transverse notch.

The propagation of the fatigue crack from the notch is observed by meansof a strain gauge and a so-called electrical method based on thevariation of resistance of the rail during the course of the progressionof the crack. One gets, by varying the amplitude of the applied stress,a series of readings at a given cumulative number of cycles and tracesthe curve of the depth of crack p against the number N of cycleseffected.

This test has been applied in a first example, to two samples of a UIC60 rail of naturally hard grade B, taken from the same bar, one samplehaving been roller straightened, the other stretch straightened. FIG. 6ashows that the roller straightened rail has a rather narrow fatiguecrack area scattered with brittle pops; FIG. 6b shows the face of astretch straightened rail which shows a clearly more developed area offatigue crack, said area being free of brittle pops. Table IV belowshows that the number of cycles required to initiate the crack and thatthe number of cycles required for its propagation are, under the sametest conditions, clearly greater in the case of a stretch straightenedrail, which is an indication of better tenacity and thus increasedreliability.

                  TABLE IV                                                        ______________________________________                                                 Roller    Stretch    Difference                                               Straightening                                                                           Straightening                                                                            in %                                            ______________________________________                                        Number of cycles                                                                         350,000       500,000  142                                         for initiation                                                                Number of cycles                                                                         750,000     1,050,000  140                                         for propagation                                                               before a clean                                                                break                                                                         Critical depth                                                                           25          28         112                                         of crack in mm                                                                ______________________________________                                    

Graphs 11 and 12 of FIG. 7 show the same relation p=f(n) mentioned inTable IV. Note that the ratio: fatigue surface (stretch straightening)fatigue surface (roller straightening) is equal to 1.55.

The previously mentioned test has been carried out, in a second example,on 4 samples of a 136RE rail in a grade of steel alloyed withcrome-silicon-vanadium, having a tensile strength of 1080N/mm², takenfrom the same as rolled bar; it has been possible to compare the fatiguebehaviour in the following different states.

roller straightened

stretch straightened

not straightened (as delivered by the cooling beds)

first roller straightened and then stretch straightened.

FIG. 8a shows the semi-brittle appearance of the broken surface of theroller straightened rail where no fatigue surface can be seen; FIG. 8bshows the large fatigue surface of the stretch straightened rail. FIG.8c shows a fatigue surface of a not straightened rail, which is veryslightly smaller than the latter; FIG. 8d shows that a stretchstraightening applied after a preliminary roller straightening restoresa good fatigue appearance.

Table V below shows the very clear improvement brought about by thestretch straightening to the number of cycles for initiation, and thenumber of cycles for propagation in comparison with the rollerstraightening.

                                      TABLE V                                     __________________________________________________________________________    Roller       Not    Stretch                                                                              Roller Straightened then                           Straightening                                                                              Straightened                                                                         Straightened                                                                         Stretch Straightened                               __________________________________________________________________________    Number of                                                                           400,000                                                                                420,000                                                                              850,000                                                                            1,150,000                                          cycles for                                                                    initiation                                                                    Number of                                                                           950,000                                                                              1,500,000                                                                            1,250,000                                                                            1,400,000                                          cycles for                                                                    propagation                                                                   up to a                                                                       clean break                                                                   Critical                                                                            26     27     26     28                                                 depth of                                                                            (semi-brittle)                                                          crack                                                                         in mm.                                                                        __________________________________________________________________________

Curves 13 to 16 in FIG. 9 show the same relation p=f(n) as was mentionedin the foregoing Table V respectively for rails of a 136 RE steel androller straightened (curve 13), not straightened (curve 14), stretchstraightened (curve 15) and first roller straightened then by stretchstraightened (curve 16). It follows very clearly from Table V and curves13 to 16 of FIG. 9 that the resistance of a rail to the propagation ofcracks is improved further still when a roller straightened rail issubjected to a stretching with residual elongation according to theinvention in order to relieve the internal stresses.

The improvement in the behaviour of the rate of cracking of railsstretch straightened according to the invention is to be linked to thereduction of the residual stresses and in particular with the almostcomplete disappearance of residual traction stresses in the head of therail, which are created by the roller straightening. This reduction ofresidual stress brought about by the method of straightening accordingto the invention enables the requirements of numerous railway tracksystems to be met, in particular of the heavy haul (such as mine tracks)which consider that residual stresses are responsible for the incidenceof dangerous breaks in the track. The stretch straightening method ofthe invention considerably improves the fatigue behaviour of railscompared to that of the roller straightened rails.

Stretch straightening gives, inter alia, the advantage of raising theyield point of the metal, in contrast to the roller straightening methodwhich has the tendency to lower it; this advantage is particularlyinteresting for the head, since a higher yield strength allows it betterto resist plastic flow which could result from heavily laden wheels onthe tread surface of the rail head. This raising of the yield point forUIC 90 grades A and B of steel, AREA, and similar, is of the order of100N/mm² for 1% elongation. This property is observed in all steels,including the extra-hard alloyed or heat treated steels. The differencein the yield point between the roller straightened and the stretchstraightened rails can amount to 20%.

It has been determined that this increase of the yield point is producedwithout degradation of the criteria of plasticity (distributedelongation and striction) or of the tenacity (K_(1c), coefficient ofcritical intensity of stress).

The measurement of residual elongation on a certain number of baselengths marked along a rail has shown that the partial residualelongations measured on each of the base lengths are constant and areall equal to the global residual elongation given to the rail. No effectof localised striction on the length of the rails was noticed. Thereduction in height is uniform over all the length of the rails,likewise the reduction in width of the foot. The slight variations indimensions observed are, as in the case of roller straightening,priorily compensated for as before by an appropriate roll pass design,which allows the specified dimensional tolerances to be respected atleast as easily as with the roller straightening method. In this lattermethod, dimensional irregularities nevertheless remain because the endskeep the original as rolled dimensions.

The invention also relates to railway rails having extremely smallresidual stresses. This type of rail is still not known at the moment,for in a quite recent study (April 1981, not published, made by R.Schweitzer and W. Heller (DUISBERG-RHEINHAUSEN) and entitled"Co-efficient of critical intensity of stress, inherent tensions andresistance to break of rails") it has been stated in conclusion that " .. . it is therefore important that the inherent stresses (=residualinternal stresses) should be maintained at as low a level as possible ifone wishes to increase the tensile strength." Now, at the presentmoment, this idea is scarcely realisable, the less so because thestraightening of the rails, indispensible to achieve and set theirstraight form, results in substantial inherent tensions.

The present invention proposes rails which after straightening have lowresidual stresses which are:

lower than +/-50N/mm² (+50N/mm² in traction; -50N/mm² in compression)for rail steel grades (heat treated or not) of a tensile strengthRm≦1000N/mm²);

lower than +/-100N/mm² (+100N/mm² in traction; -100N/mm² in compression)for rail steel grades (heat treated or not) of a tensile strengthRm>1000N/mm².

What is claimed is:
 1. A method for straightening a railway rail andremoving internal stresses therefrom comprising the steps of:(a)providing a steel railway rail having an asymetrical profile andtriaxial stress distribution pattern, and (b) simultaneouslystraightening the rail and removing internal stresses therefrom throughstretching the rail by subjecting same to tensile stress exceeding theconventional 0.2% offset yield strength of the steel, and up to a stressvalue corresponding to a total plastic deformation of the whole rail. 2.The method of claim 1 wherein the rail is subjected to sufficienttensile stress to produce at least 0.3% residual elongation upon releaseof the stress.
 3. The method of claim 1 wherein the rail is subjected tosufficient tensile stress to produce a maximum 1.5% residual elongationupon release of the stress.
 4. The method of claim 1 wherein the rail issubjected to sufficient tensile stress to produce between 0.5 and 0.7%residual elongation upon release of the stress.
 5. The method of claim 1including providing a steel railway rail comprising a grade of railsteel having a tensile strength Rm lower than or equal to 1000N/mm². 6.The method of claim 1 including providing a steel railway railcomprising a grade of rail steel having a tensile strength Rm greaterthan 1000N/mm².